Cables comprising twisted pairs, for example Category 5 cable (CAT5) are widely used to transmit signals between computers and various telecommunications, networking and display devices. CAT5 comprises four unshielded or shielded twisted pairs, each of the twisted pairs comprising two individual conductors (one for running a positive signal and one for an inverted signal). Each individual conductor is twisted together to form one pair of conductors. Four pairs of conductors are bundled together to form twisted pair cable.
The present invention pertains to a method of encoding and decoding computer video signals for transmission over a twisted pair cable. More specifically, this invention allows to transmit five components of an analog video signal generated by a computer (such a s red, green, blue, horizontal sync and vertical sync components) by utilizing only three pairs of conductors. This allows to utilize the unused pair of conductors for transmitting other signals, such as audio or control signals.
Conventionally, transmitting five components of an analog video signal by utilizing only three pairs of conductors is done by first combining horizontal sync and vertical sync components (i.e. generating “composite sync”) and then combining composite sync with the green component (or other color component). After the transmission, composite sync is separated from the green component and then horizontal sync is separated from vertical sync.
This method has disadvantages, such as expensive circuitry needed to remove composite sync from the green component and then separate horizontal sync from vertical sync. Further, removing composite sync from the green component is done by clipping, which causes compression and deterioration of the green component. Removing composite sync from the green component can also be done by cutting the sync out, which introduces artifacts into the signal. Leaving sync in the green component will affect most display devices that are not designed to handle sync on green, having the effect of the picture becoming green. Separation of horizontal sync from vertical sync is also problematic due to the artifacts introduced into the horizontal sync signal.
Accordingly, there is a clear need for a method of encoding and decoding computer video signals for transmission of the five components of an analog video signal by utilizing only three pairs of conductors without the disadvantages caused by separation of composite sync from video and separation of horizontal sync from vertical sync.